Tighter floating-point type constraints in some cases

Tighter floating-point type constraints in some cases

CONSTRAIN-FLOAT-TYPE used to return a closed bound in some cases where
the corresponding (tighter) open bound would have been derivable,
leading to missed optimisation opportunities. For example the compiler
did not derive that x is not zero in the following call to LOG:

(defun foo (x)
  (declare (type (single-float 0.0) x))
  (when (> x 0.0)
    (log x)))

Fix CONSTRAIN-FLOAT-TYPE so that it returns the tightest possible result
in all cases.

See lp#894498 for details.